Plastic containers have become more popular in the beverage bottling industry in recent years for a variety of reasons. Plastic containers are lighter weight and reduce freight costs. When they are dropped on hard surfaces they do not shatter like glass, and typically do not break. They also tend to cause less wear and tear on the conveyors employed in bottling and packaging plants. Furthermore, both bottle and closures are typically reusable, but closures are typically not reused in bottling plants, but only by the consumer
Plastic containers may be made from any number of materials depending on the application. One material is polyethylene terephthalate, “PET”. Two types of PET bottles that are commonly used are single trip and multi-trip bottles. Single trip bottles are those which are filled, used, and then discarded. Multi-trip or refillable (REF-PET) bottles are collected and reused and must be washed before refilling.
REF-PET bottles generally cost more than glass containers. Recycling of PET containers was recently approved by the FDA in the US to permit turning used containers into new ones making the use of PET even more attractive. Thus, recycling used bottles has become economically attractive.
Single trip bottles are cut up and the pieces must be washed before the PET plastic is melted for molding new bottles. The cut up pieces are then washed with chemicals that do not corrode the plastic. In time, glass soft drink bottles are expected to disappear from the market. This heightens the relevance of PET container processing even further.
Cleaning of PET can be difficult, however, and one serious problem which occurs with returnable reusable PET bottles is mold, particularly in warmer climates. Rejection rates of 40–50% have occurred at certain times of the year in countries located in tropical climates. Discarding all of the bottles from which mold cannot be removed can thus be prohibitively expensive. Therefore, it is desirable to clean the PET bottles, rather than discard them.
The cleaning of PET bottles takes place over a series of steps using caustic immersion tanks and spray wash stages in a bottlewasher. In the wash tanks, product residue, dirt, labels and labeling adhesive are removed.
There are several problems associated with the cleaning of PET bottles because they cannot be washed like glass bottles. For one thing, because the surface of PET bottles is hydrophobic, cleaning them is more difficult than glass bottles. Also, the lower washing temperature decreases the chemical activity of the bottle washing solution. While glass bottles are normally washed at 80° C., PET bottles must be washed at lower temperatures of about 60° C. due to the glass transition temperature of PET. At temperatures higher than 60° C., the PET can deform and shrink. The cleaning power of a bottlewashing solution at 60° C., however, is only one quarter that at 80° C. This compounds the problem of trying to eliminate mold, as well as other microbiological forms of life such as bacteria, spores, and yeasts, for example, from the returned bottles. Bottles returned with product residue, i.e., those bottles that have not been rinsed, are almost always contaminated with microbiological forms of life.
A further problem in the cleaning of returnable PET bottles is that PET, unlike glass, has difficulty withstanding relatively high concentrations of caustic. While glass may be washed with up to 5.0% caustic, as little as 0.1 to 0.2 wt-% caustic may cause corrosion. However, concentrations of less than about 1.5% caustic are typically not practical for cleaning. Thus, when PET articles are washed for recycling and reuse, however, it is typically a more highly caustic detergent composition that is used to remove old labels and to clean and sterilize the interior of the articles because lower concentrations tend to be less effective at attacking and removing soil.
Highly caustic solutions can also cause what is referred to in the industry as “stress cracks” in the PET. Highly caustic solutions tend to attack and degrade PET which can lead to “stress cracks” within, or even completely through the walls of the articles over repeated washing cycles, or even within a single washing cycle. Conventional caustic bottle washing compositions often also contain other constituents which have a deleterious effect on PET.
Indeed, it has long been known that exposure by such articles to these compositions leads to these phenomenon which has been identified as “stress cracking” in these PET containers and other such articles of manufacture. “A general surface attack can result in “fogging” of the normally transparent PET material. This is a result of chemical etching of the surface of the PET container by the caustic present in the wash bath and is commonly referred to in the industry as “hazing”.
As noted, conventional aqueous-based bottlewashing compositions containing caustics, alcohols, nonionic surfactants and/or other additives do not inhibit or prevent stress cracking in such containers, but rather, promote stress cracking.
Stress cracking can cause loss of carbonation pressure and ultimately can result in product loss from the container. Stress cracking may further lead to bottles which break. This can result in downtime for cleaning. Furthermore, when cases are stacked, other bottles can become sticky from the spilled beverage, in the case of sodas, for example.
Thus, with glass, the washing temperature, the caustic concentration, and the washing time may be adjusted to allow for variability within the environment in contrast to PET containers which cannot withstand high levels of any of these variables.
While various alternatives have been proposed such as lowering the level of caustic, there remains a need in the industry for compositions and methods which allow for the efficient cleaning of PET containers and multiple reuse events.